Amino Acids and Proteins: Characteristics and Structure

Protein Structure and Function

• The three-dimensional structure of a multisubunit protein is called a quaternary structure.
• Denaturing agents like trichloroacetic acid, phenol, and acetone can lead to protein precipitation.
• The process of refolding an unfolded (denatured) protein to restore its native structure and function is called renaturation.

Protein Composition and Formation

• Proteins consist of two separate polypeptide chains or subunits.
• About 15-20% of the body weight consists of proteins.
• Proteins are formed from amino acids.
• The amino acid sequence of a protein depends on the sequence of nucleotides in the gene that encodes it.

Amino Acid Properties

• Amino acids are divided into two main classes based on their properties: polar and nonpolar.
• Nonpolar amino acids constitute the hydrophobic core of the protein's structure.
• Polar amino acids tend to be located on the surface of a protein's structure.

Functional Groups and Bonds

• The amino and carboxyl functional groups in amino acids function as weak acids and bases.
• A peptide bond is formed by removal of the elements of water between two amino acid molecules.

Protein Structure Hierarchy

• The primary structure of a protein is the sequence of amino acids.
• The secondary structure that involves a tightly wound helical conformation of a polypeptide chain is an α-helix.
• Hydrogen bonds stabilize the structure of an α-helix in proteins.
• The secondary structure that has an extended conformation of polypeptide chains arranged into a zigzag is a β-pleated sheet.
• In a parallel β-pleated sheet, the polypeptide chains run in the same direction.
• A β-turn involves four amino acid residues and is important in compacted globular proteins.
• The tertiary structure of a protein reflects the overall 3D shape of a polypeptide chain.

Chirality and Ions

• An asymmetric carbon atom is found in amino acids with chiral centers.
• Enantiomers are compounds that are non-superposable mirror images of each other.
• Zwitterions or dipolar ions have both positive and negative charges.